Interpretive Summary: Raman chemical imaging combines Raman spectroscopy and digital imaging to map the composition and morphology of a target. Although highly effective for chemical detection and food safety applications such as ingredient authentication, current commercially available Raman chemical imaging systems are not suitable for use in screening larger sample areas, due to either the limitations of the microscopy-based imaging systems or the long data acquisition times required by the larger point-scan imaging systems. This article reports the development of a new line-scan Raman chemical imaging system that is capable of scanning larger, non-microscopic sample areas at speeds significantly faster than those of current systems. This line-scan hyperspectral Raman imaging system demonstrated imaging of four Petri dishes positioned within a 20cm x 47mm area in only 4 minutes compared to the 10 hours required by our previously developed point-scan Raman system, acquiring one spectrum for each pixel in the 512×110 pixel image. In our illustrative example, Raman chemical images were created for detecting the presence of two adulterants, melamine and dicyandiamide, mixed together into samples of dry milk powder that were contained in the Petri dishes. Line-scan Raman chemical imaging has great potential for use as a screening or authentication tool for food safety applications and we feel that the development of this new line-scan Raman chemical imaging system will greatly impact and benefit the spectral imaging field and the food industry.

Technical Abstract:
A line-scan hyperspectral system was developed to enable Raman chemical imaging for large sample areas. A custom-designed 785 nm line-laser, based on a scanning mirror, serves as an excitation source. A 45° dichroic beamsplitter reflects the laser light to form a 24 cm × 1 mm excitation line normally incident on the sample surface. Raman signals along the laser line are collected by a detection module consisting of a dispersive imaging spectrograph and a CCD camera. A hypercube is accumulated line by line as a motorized table moves the samples transversely through the laser line. The system can measure Raman shifts up to 2889 wavenumbers, for a 23-cm linear field of view. An example application for the authentication of milk powder was presented to demonstrate the system performance. In four minutes, the system acquired a 512×110×1024 hypercube (56,320 spectra) from four 47-mm-diameter Petri dishes containing four powder samples. Chemical images were created for detecting two adulterants (melamine and dicyandiamide) that had been mixed into the milk powder.